In my opinion, Jenny’s departure from Forrest’s house, AND her “No” answer to his proposal for marriage are inextricably linked.
It’s obvious that Jenny and Forrest loved each other as best friends from the time they first met. There was absolutely nothing that one of them could do that would break that bond with the other.
But did Jenny love Forrest like a woman would love a husband? Yes is my vote, and here is why. From Day 1, Jenny saw into Forrest’s heart and soul – beyond leg braces and beyond low IQ – and saw a good, good man. So why say “No” to his proposal? It’s because she did not see herself as worthy of being Forrest’s wife. She is the only one in the world who knows just how much her life’s conduct has driven her away (and below) from being a worthy wife. Please look at the dialog between them at his proposal, or view this YouTube clip.
Forrest: Will you marry me?
(Jenny slowly turns around to face Forrest without expression.)
Forrest: I’d make a good husband, Jenny.
Jenny: (Matter-of-factly) You would, Forrest. [Emphasis mine. She means it – this is no kidding.]
Forrest: But you won’t marry me.
(Jenny shakes her head no with a disappointed sigh and expression)
Jenny: (Shaking her head no) You don’t want to marry me. [Emphasis mine – this is crucial.]
Forrest: Why don’t you love me, Jenny? [Forrest has it wrong.]
(Jenny goes to respond, but doesn’t find the words.) [Because she is conflicted.]
Forrest: I’m not a smart man. But I know what love is. (Walks away.)
Later on in the clip, Jenny comes to Forrest’s room:
Forrest: (Surprised) Jenny?!
Jenny: Forrest I do love you. [Emphasis mine]
(No more dialog as this is where Little Forrest is conceived.)
Aside from Forrest’s understandable surprise, these are exactly the actions that a husband and wife would do. Jenny knows all too well how to practice birth control. And she knows all too well that she is not doing so. She owned up to the only part of being a wife that she would allow herself to attain.
So why did she leave?
As loving friends, she probably would have stayed quite a while. As a prospective wife, she could not bear to be in his presence. She did not see herself as being in his league. In her mind, she had to leave. That was her loving gesture to him so that he could find someone who was worthy of him. She knew he would not understand, so she left in a manner that would cause the least pain to him.
Unfortunately, it’s only until she is dying that she realizes that Forrest sees her heart and soul as much as she sees his. It’s only after he offers to take care of her that she realizes that that’s all he sees: a great person that has always been worthy to be his wife. And that is why she proposes to him. Of course, he accepts.